一种计算页岩岩心解吸测试中损失气量的新方法

目前，页岩岩心解吸测试过程中，损失气量的计算仍然借鉴煤层岩心解吸测试中所采用的解析法。通过对页岩岩心实测解吸数据的分析，发现部分页岩岩心取心时间长，岩心存在天然裂缝导致孔渗不均一性较强（解吸曲线存在多个直线阶段），对这些岩心进行解吸测试，其损失气量的计算不符合解析法计算损失气量的条件。鉴于页岩岩心罐装解吸测试过程与封闭外边界定压开采的气井生产过程具有一定的相似性，由此提出采用Arps递减法计算损失气量。采用3种Arps递减方式对页岩解吸曲线进行拟合，发现指数递减法对整个测试区间内的实测数据拟合效果最好；对比解析法和Arps指数递减法对损失气量的计算结果，发现解析法低估了20%~40%的损失气量。结论认为：Arps指数递减法是采用经验公式对岩心解吸实测数据进行拟合，能够反映多种综合因素对页岩岩心解吸速率的变化规律；其所采用的参数较少，并且不需要苛刻的前提条件，更适用于页岩岩心解吸测试中损失气量的计算。

A new method of calculating the lost gas volume during the shale core desorption test

The shale gas volume lost during the shale core desorption test is calculated by the analytical method that is regularly used in the coal bed methane desorption test at present. However, the analysis of the measured desorption test data of shale cores reveals that the time of shale coring is usually long and the heterogeneity of porosity and permeability is strong due to the existence of natural fractures in cores (multiple straight lines in the desorption curves).These characteristics make this analytical method unsuitable to calculate the lost shale gas volume. Therefore, as the shale core canister desorption test process is similar to some extent to gas well recovery process with constant pressure under closed outer boundary, the Arps decline method was then proposed to calculate the lost shale gas volume in the desorption test. Three Arps decline patterns were used to match shale core desorption curves and the exponential decline method was found to be the best in matching with the measured data in the whole test interval. As a result, 20% 40% of the lost shale gas volume, underestimated by the previous analytical method, was calculated by the Arps exponential decline method. In conclusion, this new method, with few parameters needed and restrict prerequisite unnecessary, can reflect the influences of various factors on the shale core desorption, and is more suitable for calculating the lost gas volume during the shale core desorption test.